1. Field of the Invention
The present invention relates to an arrangement for the catalytic oxidation of environmentally harmful components in a cooled carrier gas of a chemical-technical or physical-technical process. In the arrangement, the purified gas to be conducted into the atmosphere is in heat-exchanging contact through a multipass heat exchanger with the carrier gas upstream of the catalyst.
2. Description of the Related Art
In a number of technical processes, for example, in the manufacture of phthalic anhydride or maleic anhydride, the desired product cannot be completely obtained from the carrier gas. A small amount of the product remains in the carrier gas. Also remaining in the carrier gas are hydrocarbon portions which cannot be converted in the process. The hydrocarbon portions reach together with the carrier gas the atmosphere through the stack of the plant used in the chemical-technical or physical-technical process and represent a burden on the environment.
Because of the continuously increasing awareness with respect to environmental protection, the regulations of many countries concerning emissions require that the abovementioned product residues as well as the hydrocarbon portions are removed before the carrier gas reaches the atmosphere in order to avoid harmful influences on the environment. In the past, thermal combustion and catalytic oxidation were available for this purpose.
Although the product residues as well as the hydrocarbon portions are burned without problems in the thermal combustion because of the high combustion temperatures, there is still the problem that simultaneously a large quantity of nitric oxides are produced which are also harmful to the environment. The reason for the production of nitric oxide is the high nitrogen portion contained in the combustion air. As a consequence, in actual practice the catalytic oxidation which is conducted at lower temperatures is used more and more because this method removes the product residues as well as the hydrocarbon portions, while not releasing any additional nitric oxides.
In a known arrangement for the catalytic oxidation, the carrier gas which contains product residues as well as hydrocarbon portions is in heat-exchanging contact befor being admitted to the catalyst with the purified gas discharged from the catalyst, in order to reach the so-called start-up temperature of the catalyst. In other words, the temperature increase of the purified gas resulting from oxidation is utilized for raising the carrier gas to the necessary start-up temperature by means of a heat exchange with the carrier gas. For this purpose, a single-stage multipass heat exchanger is used in which the carrier gas is conducted in a transverse flow to the purified gas.
However, such a heat exchanger has the disadvantage that the expenses are very high for conducting the carrier gas and the purified gas in a channeled manner because of the necessary deflections of the gas ducts upstream and downstream of the heat exchanger. This is particularly significant if it is considered that not infrequently volumetric flows of 100,000 Nm:/h must be handled.
Another deficiency of the known arrangement resides in the fact that frequently interruptions of the operation take place which endanger the overall process. The reason for these interruptions in the operation is the fully saturated carrier gas which emanates from the main process and is laden with residue materials. The carrier ga is at the dew point with respect to the precipitated product. As a result, it cannot be avoided that small amounts of the precipitated products are conducted along in finely distributed drop-shaped form or in solid form and are deposited on the surfaces of the heat exchanger. In addition, corrosion and/or cloggings may occur.